Background: Despite effective HIV prevention and treatment, the global HIV epidemic continues to grow. Current programs to engage populations at high HIV risk have had limited impact, in part because they do not reach those at highest risk. To reduce the global burden of HIV, new, evidence-based, far-reaching strategies are needed to engage individuals at risk in HIV prevention and treatment. In the U.S. and globally, people are increasingly using mobile/social technologies (SMT) (e.g. dating apps, WhatsApp, Facebook) to socialize and meet partners. While this shift in how individuals socialize has disrupted traditional face-to-face outreach, SMT provides an unprecedented opportunity to engage historically hard-to-reach groups, such as men who have sex with men (MSM), for HIV prevention and treatment. Th goal of this study is to test whether a SMT-based behavioral intervention that includes a digital coupon for free HIV testing (CHALO! 2.0) enhances HIV testing and linkage-to-care among a population that continues to experience high rates of HIV infection in India. The scientific premise of this proposal is that a community-developed, theory-based intervention will engage MSM at high risk of HIV who are reached online, and that the intervention will increase HIV knowledge and enhance motivation and behavioral skills to obtain HIV testing and get linked to HIV prevention (if HIV- uninfected) or HIV treatment (if HIV-infected). The rationale for this research study is that determining efficacy of a rapidly scalable SMT-based behavioral intervention is needed before widespread dissemination. The significance of this research question is that even a modestly effective SMT-based HIV prevention intervention may efficiently reach many individuals at high risk of HIV and lead to increased HIV status awareness, better linkage-to-care, and fewer new HIV infections. Methods: In a pragmatic randomized controlled trial, we will determine whether CHALO! 2.0 results in increased HIV testing and linkage-to-care (prevention or treatment) among sexually active MSM who are unaware of their HIV status (never tested or tested >6 months ago) and are recruited online. We will conduct a 12-week, three arm randomized trial comparing CHALO! 2.0 to two control conditions: an attention-matched SMT-based control (also including a digital coupon for free HIV testing), and a digital coupon only control. Potential Impact: While global health agencies have called for online interventions to engage populations at high HIV risk who are not being reached, few effective models exist and none exist in India, which is home to one of the world?s largest MSM HIV epidemics. This study will address this gap by testing a scalable and replicable online model to engage individuals at high HIV risk and link them to HIV testing and prevention or treatment. This proposal is responsive to NIH PA-18-278: Innovations in HIV Testing?to Optimize HIV Care Continuum Outcomes, and addresses multiple high priority research areas of NIH?s Office of AIDS Research. The proposed model, if found effective, may also accelerate achieving UNAIDS 90-90-90 care targets.